1. FIELD OF THE INVENTION
This invention relates to insulating tubular conduits for use, especially in subterranean oil wells, and to an indirect method of inspecting and testing insulating tubular conduits to determine if their insulating performance is satisfactory.
2. DESCRIPTION OF THE PRIOR ART
One technique for recovering viscous oil deposits is to heat the crude deposits thus reducing their viscosity and permitting them to flow to the surface. One method of heating the subterranean deposits is to inject superheated steam through an injection well communicating with the viscous crude. The heated oil can then flow to the surface in the injection well or in other wells communicating with the producing formation. Although not new, such steam injection techniques have not heretofore been commercially competitive because of the relatively high cost of steam injection equipment and because of the large heat loss which may occur between the point at which the super heated steam is injected and the formation.
One means of reducing the transmission heat losses is the use of concentric wall insulated tubing. Examples of concentric wall insulated tubing are shown in U.S. Pat. No. 3,511,282 and the insulated tubing design sold by the General Electric Company under the trademark Thermocase. A concentric wall thermal insulated conduit is also disclosed and claimed in U.S. patent application Ser. No. 272,411 filed on June 10, 1981. Each of these designs employs a plurality of individual tubular sections which, when attached end to end in tandem relationship, form a fluid transmission conduit or tubing string. Each of these designs also employs inner and outer tubular members attached adjacent their ends to define an annular insulating cavity extending along substantially the entire length of each section. Conductive, convective and radient insulating barriers or material may be located within each insulated cavity. Inert gases resistant to the transmission of heat through the cavity may also be employed in the cavity. Since the cavities are hermetically sealed at least a partial vacuum may be established within the annular insulating cavity to prevent heat loss.
As with many tools used in subterranean wells, concentric wall insulated tubing can be expected to undergo degradation during its useful life and the insulating properties of all types of tubing sections can be expected to deteriorate. Continuous steam injection can be expected to result in a reduction of the insulating performance of any insulating tubing design before the tubing members themselves become unusable. The tubing members represent a significant part of the cost of an insulating tubing joint. One significant problem would involve the deterioration of the welds or other means of sealing the annular insulating cavities at the points where the inner and outer tubing are attached to each other. Given the fact that the tubing is used for steam injection, it could be expected that a large amount of moisture would develop in the annular insulating cavity if the welds deteriorated.
Since it would be desirable to reuse the rather expensive insulating tubing sections and because the tubing members themselves would not be expected to deteriorate before the insulating performance reached an unacceptable level, some method of non-destructive testing to determine insulating performance is necessary. Ideally a method of on-site testing which can be utilized each time the tubing sections or joints are removed from the well would be useful. One method of indirectly determining the insulating performance of an insulating tubular member is to utilize temperature recorders affixed to the exterior of the outer tubular member in each section. These temperature recorders, of the type shown in U.S. Pat. No. 3,002,385, are of such a composition as to change, color when the temperature reaches a certain level. Thus removal of the tubing and monitoring of these temperature recorders will indicate the maximum temperature of the outer tubing member. When the temperature has reached an undesirable level it would be assumed that the insulating performance of that particular joint is defective. However, this method assumes that the heat transfer causing the outer tubular section to reach an elevated temperature is due to heat transfer through the annular insulating cavity. There are, however, other components in a fluid transmission conduit or well completion which could cause the outer tubing member to reach undesirable elevated temperatures. For example, a leak in the coupling member between joints could expose the tubing members to steam which has leaked through the coupling to the annulus. A defective or leaking packer would also permit steam to rise from the producing formation into the annulus above the packer and would also result in a faulty indication that the tubing joints were defective. The non-destructive testing and inspection method disclosed herein allows indirect evaluation of the heat transfer through the annular insulating cavity of each tubing section, without the introduction of error due to leaks in the tubing coupling, the packer, or other components of the fluid transmission conduit.